Hubble's
Law
During the 1920's an American
astronomer Edwin Hubble (1899-1953) made a very important discovery. Hubble
noticed that the spectral lines of distant galaxies were red-shifted, indicating
that they were moving away from us. Using standard candle methods as distance
indicators, he established the distances to a number of galaxies for which
the radial velocities had been measured using the Doppler formula. Hubble
plotted a graph of radial velocity versus distance and obtained a straight
line which showed that radial velocity was directly proportional to the
distance. The result is known as the Hubble Law
v=H0r
where H0 is known as
the Hubble constant with unit of km s-1 Mpc-1,
if v is measured in km s-1 and distance r in Mpc.
The value of the Hubble "constant" is the subject of much debate. The current
value is thought to lie between 50 and 100 km s-1 Mpc-1.
Given the radial velocity, the Hubble Law becomes a technique for measuring distances.
The distances measured using the Hubble Law depend on how well we know
the value of H0. Astronomers can measure the distances of galaxies
by independent methods up to a distances of about 400 Mpc. Beyond this
we can not be sure that the Hubble law remains true; in fact there are
reasons to think that it departs from a straight line at very large distances,
and astronomers devote much experimental effort in determining an accurate
value for the Hubble constant.
The
Cosmological Red shift
An expanding universe explains
why light from distant galaxies is red-shifted. Cosmologists distinguish
between a red shift caused by the Doppler effect and one that is caused
by the expansion of the universe. Although the Doppler formula is used
in both cases to calculate velocities, it is important to understand that
a Doppler shift is caused by a body's motion through space whereas
a cosmological red shift is caused by the expansion of space.
EDWIN HUBBLE
